Compression Molding (Composites)

Compression molding forms composite parts by pressing charge material in a heated matched mold, delivering repeatable near-net shapes at production rates.

Overview

Compression molding (composites) makes parts by placing a composite charge—often SMC/BMC or prepreg—in a heated matched-metal tool and closing the press to consolidate and cure. It produces consistent thickness, good surface finish on mold faces, and repeatable fiber/resin content compared with open-mold methods.

Choose it for medium-to-high volumes where tool cost is justified and geometry fits a two-sided mold with controlled draw. Cycle times are typically minutes, and automation is practical for loading/unloading. Tradeoffs: limited deep undercuts, inserts and ribs require careful flow/venting design, and cosmetic quality is best on mold-contact surfaces. Parts often need edge trimming and drilled features; tight flatness and warpage control depend on layup, cure control, and symmetric laminates. Best fit: structural covers, brackets, shells, and panel-like components needing high repeatability.

Common Materials

  • SMC (glass/polyester)
  • BMC
  • Carbon/epoxy prepreg
  • Glass/epoxy prepreg
  • Carbon/PEEK prepreg
  • Phenolic prepreg

Tolerances

±0.010" to ±0.020"

Applications

  • Automotive body panels (SMC)
  • EV battery enclosures and lids
  • Electrical insulation housings (BMC)
  • Aerospace access panels
  • Structural brackets and stiffened covers
  • Sporting goods shells and guards

When to Choose Compression Molding (Composites)

Pick compression molding when you need repeatable composite parts at medium-to-high volumes with short cycle times and controlled thickness. It fits parts that can be formed in a matched tool with manageable draft, limited undercuts, and predictable trim operations.

vs Resin Transfer Molding

Choose composite compression molding when you need faster cycles and higher throughput with highly repeatable thickness and surface on both mold faces. It’s a strong fit for SMC/BMC-style charge molding where resin metering and injection hardware isn’t desired.

vs Vacuum-Assisted Resin Transfer (VARTM)

Choose composite compression molding when you need production-rate curing and tighter control of fiber volume/resin content than vacuum-only consolidation can typically deliver. It also reduces variability from bagging, consumables, and operator technique at higher volumes.

vs Prepreg Layup with Autoclave

Choose composite compression molding when you can accept modestly looser tolerances/flatness than autoclave-quality structures but need much higher throughput and lower per-part labor. Matched tooling also improves repeatability versus hand layup inside autoclave bags.

vs Hand Lay-Up

Choose composite compression molding when repeatability, cycle time, and surface consistency matter and volumes justify tooling. It reduces operator-driven variability and typically cuts post-cure finishing and rework.

vs Pultrusion

Choose composite compression molding when the part isn’t a constant cross-section and needs local thickness changes, ribs, bosses, or 3D contours. Compression molding supports near-net-shape 3D geometry that pultrusion can’t produce.

Design Considerations

  • Add draft on all draw surfaces (typically 1–3°) and avoid true undercuts unless you budget for side actions or secondary bonding
  • Keep wall thickness as uniform as possible; use generous transitions and radii to reduce knit lines, sink, and warpage
  • Define cosmetic surfaces and texture requirements by face; mold-contact faces finish best, non-contact/trim edges may need secondary finishing
  • Design ribs/bosses with realistic flow paths and venting; provide room for flash lands and controlled parting lines
  • Call out critical datums and allow for post-mold machining of holes/slots; avoid using as-molded edges as precision references
  • Specify fiber architecture (SMC type or prepreg ply schedule), resin system, and cure requirements clearly to enable accurate quoting and consistent properties